1. Field of the Invention
The present invention relates to a ceramic heater having a ceramic substrate and a heating element provided on a surface thereof, and more particularly, it relates to a ceramic heater provided with a heating element having excellent adhesion.
2. Description of the Prior Art
A ceramic heater having a substrate of ceramics provided with a heating element and a feed electrode of metals on a surface thereof is known as a heater for an electric heater, an iron or an electric stove. The substrate for such a ceramic heater is generally prepared from alumina (Al.sub.2 O.sub.3).
An alumina substrate is inferior in thermal shock resistance although it is excellent in electric insulation and mechanical strength and has a low cost. In a heater requiring rapid heating and cooling, therefore, the alumina substrate is disadvantageously broken by a thermal shock and exhibits inferior reliability in actual use. In the alumina substrate, further, a remarkable temperature difference is caused between a portion provided with the heating element and the remaining portion due to small thermal conductivity of about 20 W/m.multidot.K. Thus, the alumina substrate is unsuitable for a heater requiring homogeneity of temperature distribution, i.e., thermal homogeneity.
In order to solve such problems of the alumina substrate, a ceramic heater employing a substrate consisting of aluminum nitride (AlN) has been proposed. For example, Japanese Patent Laying-Open No. 4-206185 (1992) discloses an aluminum nitride heater employing paste of Pd and Pt and a method of preparing the same. Japanese Patent Publication No. 7-109789 (1995) (Japanese Patent Laying-Open No. 62-229782) proposes an aluminum nitride heater employing a metal having a high melting point as the material for a heating element.
As hereinabove described, a ceramic heater employing an aluminum nitride substrate having excellent thermal conductivity is superior in thermal homogeneity with improved thermal shock resistance of the substrate. When the aforementioned heating element of Pd and Pt or a metal having a high melting point or a well-known heating element of Ag or an Ag alloy is formed on a surface of the aluminum nitride substrate, however, the ceramic heater is deteriorated in reliability due to insufficient adhesion between the heating element and the substrate.
In the heater described in Japanese Patent Laying-Open No. 4-206185, the manufacturing cost is remarkably increased due to the heating element of Pt and Pd. To this end, Japanese Patent Publication No. 7-109789 or the like proposes a heating element prepared from a metal having a high melting point or an active metal.
When the heating element is made of a metal having a high melting point, however, the substrate is warped or deformed if the aluminum nitride forming the substrate and the metal having a high melting point are fired at the same time due to a difference between the respective shrinkage ratios of the aluminum nitride and the metal having a high melting point during sintering. In order to solve this problem, the metal having a high melting point is printed on the aluminum nitride sintered body and thereafter fired. In this case, however, the manufacturing cost is increased due to two steps of firing and it is still difficult to completely prevent warpage or deformation of the substrate. When the heating element is made of an active metal, on the other hand, a high vacuum is required for formation thereof, to disadvantageously result in a high manufacturing cost.